linux-kernel - [PATCH] pci/pcie: Avoid unnecessary PCIe link retrains

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Date:	Tue, 3 Nov 2009 16:38:20 -0500
From:	RDH <rdh@...t.sun.com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] pci/pcie: Avoid unnecessary PCIe link retrains


This patch avoids unnecessary PCIe link retraining sequences within
the PCIe ASPM common clock setup code by issuing a link retrain
command only if we are actually changing the PCIe clock configuration.

Signed-off-by: Robert D. Houk <rdh@...t.Sun.COM>
---
 aspm.c |   20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)
--- a/drivers/pci/pcie/aspm.c	2009-10-15 20:41:50.000000000 -0400
+++ b/drivers/pci/pcie/aspm.c	2009-11-02 14:29:35.000000000 -0500
@@ -183,6 +183,7 @@ static void pcie_aspm_configure_common_c
 {
 	int ppos, cpos, same_clock = 1;
 	u16 reg16, parent_reg, child_reg[8];
+	u16 lnkctl_ccc_or, lnkctl_ccc_and;
 	unsigned long start_jiffies;
 	struct pci_dev *child, *parent = link->pdev;
 	struct pci_bus *linkbus = parent->subordinate;
@@ -205,6 +206,25 @@ static void pcie_aspm_configure_common_c
 	if (!(reg16 & PCI_EXP_LNKSTA_SLC))
 		same_clock = 0;
 
+	/* Check upstream component for Common Clock Config */
+	pci_read_config_word(parent, ppos + PCI_EXP_LNKCTL, &reg16);
+	lnkctl_ccc_and = lnkctl_ccc_or = (reg16 & PCI_EXP_LNKCTL_CCC);
+
+	/* Scan downstream component for CCC, all functions */
+	list_for_each_entry(child, &linkbus->devices, bus_list) {
+		cpos = pci_find_capability(child, PCI_CAP_ID_EXP);
+		pci_read_config_word(child, cpos + PCI_EXP_LNKCTL, &reg16);
+		lnkctl_ccc_and &= (reg16 & PCI_EXP_LNKCTL_CCC);
+		lnkctl_ccc_or |= (reg16 & PCI_EXP_LNKCTL_CCC);
+	}
+
+	/* If we want Common Clock OFF and no device/function has it on */
+	/* or we want Common Clock ON and every device/function has it on */
+	/* then there is no need to retrain PCIe links */
+	if (((same_clock == 0) && (lnkctl_ccc_or == 0))
+            || ((same_clock == 1) && (lnkctl_ccc_and == PCI_EXP_LNKCTL_CCC)))
+		return;		/* Don't retrain link(s) */
+
 	/* Configure downstream component, all functions */
 	list_for_each_entry(child, &linkbus->devices, bus_list) {
 		cpos = pci_find_capability(child, PCI_CAP_ID_EXP);
--
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